A vibration testing apparatus, sometimes known as a shaker, is employed in industry for the vibration testing of industrial elements and components. The object of such vibration testing apparatus is to expose scientific or industrial equipment to vibration of the frequency and amplitude to which that equipment may be exposed in normal use, to test the response of the equipment to the vibrations.
The present invention is particularly concerned with the vibration testing of satellites. These satellites are typically exposed to very severe vibrations during launch. These vibrations may have frequencies in the range 5–150 Hz.
A vibration testing apparatus is shown in U.S. Pat. No. 4,489,612. This includes an electromagnetic vibration generator which is mounted on a trunion which is arranged, in one configuration, to vibrate a slip plate in a horizontal plane. The slip plate is mounted on a massive support block through hydrostatic bearings. Hydrostatic bearings are employed because they give the lowest distortion and the highest stiffness to the bearing.
The electromagnetic vibration generator can also be mounted in a configuration in which the armature is oriented vertically and can supply vibration to an object mounted directly on the armature. Where the object is of a larger size than the armature, a so-called head expander maybe used, for example as disclosed in GB2211268.
In order to test the vibration modes of the equipment being tested in the frequency range desired, it is preferred to minimise vibration modes of the combination of the vibration testing apparatus and the equipment in this range.
It is normal to design the weight, dimensions and stiffness of various parts of the vibration testing apparatus so that this object is attained.
Vibration modes due to the vibration testing apparatus itself may include flexible vibration modes, for example due to the twisting of the plate. Normally, these vibration modes are of relatively high frequency, falling outside the testing frequency range. However, rigid vibration modes of the head expander, caused by rocking of the head expander within the support structure may be encountered. These may have a frequency which is lower than the flexible modes and may fall within the testing range.
This may be a problem particularly when the equipment being tested has very large dimensions, in particular extending for a large distance away from the expander or when the equipment being tested has a large mass.
Whereas it is known that placing the bearings further apart helps to increase the frequency of rigid vibration modes, this may not be possible because larger dimension mountings have increased mass and hence decreased rigid vibration modes or may be inadequately stiff and have decreased flexible vibration modes.
It is an object of the present invention to provide vibration-testing apparatus in which the amplitude and/or frequency or vibration modes of the combination of the equipment being tested and the vibration testing apparatus can be improved.